The c-kit gene encodes a receptor tyrosine kinase and the ligand for the c-kit receptor is called the stem cell factor (SCF), which is the principal growth factor for mast cells. The activity of the c-kit receptor protein tyrosine kinase is regulated in normal cells, and the normal functional activity of the c-kit gene product is essential for maintenance of normal hematopoeisis, melanogenesis, genetogensis, and growth and differentiation of mast cells. Mutations that cause constitutive activation of c-kit kinase activity in the absence of SCF binding are implicated in various diseases including malignant human cancers.